Stefan Kanfer has a good essay posted on this at Minding the Campus.
Kanfer’s key point is that if professors really wanted to catch student cheaters, they could. Most don’t “because the institution for which they work needs money, i.e. students who will come up with the fees for tuition, and who will eventually, after getting the jobs they were after, kick in to the alumni fund.” The erosion of standards has helped many schools grow fat and happy. Pushing back to restore standards, including honesty about submitted work, would jeopardize the comfortable status quo.